Phase transition in Tl2TeO3: influence and origin of the thallium lone pair distortion

A new modification of thallium tellurite, beta-Tl(2)TeO(3), has been synthesized by methanolothermal reaction, and its phase transition has been studied by single-crystal X-ray diffraction. At a temperature of 440(10) degrees C an irreversible phase transition from a monoclinic structure (beta-Tl(2)TeO(3): P2(1)/c (No. 14), Z = 4, a = 8.9752(18) A, b = 4.8534(6) A, c = 11.884(2) A, beta = 109.67(2) degrees, V = 487.47(15) A3 at 25 degrees C) to an orthorhombic structure (alpha-Tl(2)TeO(3): Pban (No. 50), Z = 8, a = 16.646(2) A, b = 11.094(2) A, c = 5.2417(8) A, V = 968.0(3) A3 at 25 degrees C) is observed. Both structures are characterized by psi-tetrahedral TeO(3)(2-) anions. In the orthorhombic structure psi-trigonal bipyramidal [TlO(4)] units are found together with psi-tetrahedral [TlO(3)] units whereas in the monoclinic structure the coordination polyhedron around Tl(I) can be best described as a psi-square pyramide, psi-[TlO(4)]. The electronic structure of Tl(2)TeO(3) in both modifications has been studied to explain the influence of the lone pairs. It can be conclusively shown that the minimization of antibonding ns-metal/2p-oxygen interactions is the driving force for "lone pair" distortions which determines the structures of Tl(2)TeO(3).     

Counterion Effect in Cobaltate-Catalyzed Alkene Hydrogenation

We show that countercations exert a remarkable influence on the ability of anionic cobaltate salts to catalyze challenging alkene hydrogenations. An evaluation of the catalytic properties of [Cat][Co(η⁴-cod)₂] (Cat=K ( 1 ), Na ( 2 ), Li ( 3 ), (^Depnacnac)Mg ( 4 ), and N(ⁿBu)₄ ( 5 ); cod=1,5-cyclooctadiene, ^Depnacnac={2,6-Et₂C₆H₃NC(CH₃)}₂CH)]) demonstrated that the lithium salt 3 and magnesium salt 4 drastically outperform the other catalysts. Complex 4 was the most active catalyst, which readily promotes the hydrogenation of highly congested alkenes under mild conditions. A plausible catalytic mechanism is proposed based on density functional theory (DFT) investigations. Furthermore, combined molecular dynamics (MD) simulation and DFT studies were used to examine the turnover-limiting migratory insertion step. The results of these studies suggest an active co-catalytic role of the counterion in the hydrogenation reaction through the coordination to cobalt hydride intermediates.     

HUNTER: A conformational search program for acyclic to polycyclic molecules with special emphasis on stereochemistry

A new conformational search program, HUNTER, connected with the force fields MMP2 and MM3(92) is presented. The program accepts all types of molecules with most different substructures, considers stereochemical facts, and covers conformational space efficiently and completely. The most important facilities are an automated analysis of the stereochemistry including topographical facts, a separate perturbation of the acyclic and cyclic parts of the molecule using modified corner flapping, and an incremental rotation around single bonds with fixed flap and rotation angles, respectively; an exclusion of high energy structures by simulated annealing; the choice of the conformer lowest in energy, which is new as an initial structure for the next sampling run; and the use of a reduced set of dihedral angles to define a conformation. A specifically devised graphic interface, SERVANT, is used to feed in and control all informations necessary for a program run and to visualize the results. Most of the parameters are user-defined and thereby allow a flexible search, including a search for the most stable diastereomer. The efficiency of the different parameter sets was tested in calculation with cycloundecane ( 12 ), (Z)-oct-3-ene ( 13 ), and sipholenol-A monoacetate ( 14 ). The best performance regarding the number of different low-energy conformers was achieved with 60° ( 14 ) and 90° flaps ( 12 ), respectively, including substituent correction for the cyclic parts, and with 105° ( 14 ) and 120° rotations ( 13 ), respectively, for the acyclic parts. In comparison to the stochastic search routine implemented in MM3(92), HUNTER performed two ( 12 ) to six ( 14 ) times better. © 1997 John Wiley & Sons, Inc. J Comput Chem 18: 1264–1281, 1997     

Fast,approximate algorithm for detection of solvent-inaccessible atoms

Up to about half of the atoms in biopolymers are inaccessible to solvents. If such atoms can be rapidly identified, time can be saved in the subsequent computation of atomic surface areas. A quick, approximate method, termed buried atom elimination (BAE), was developed for the detection of such atoms. Following the literature, the method makes use of a Gaussian function to calculate the neighbor density in four tetrahedral directions in 3-dimensional space, sometimes twice with different orientations. In macromolecules, our method detects between 63 and 81% of the buried atoms but also incorrectly classifies 2–8% of the exposed atoms as buried. These misidentified atoms all have small solvent-exposed (accessible) surface areas (SASAs): their surfaces sum to a maximum of 0.5% of the molecular SASA, and their maximum atomic SASA is 5.1 Ų. Using our recently reported LCPO method for computing atomic surfaces, which is one of the fastest available, the use of BAE increases the overall speed of computing the atomic SASAs by a factor of up to 1.6 for surfaces only and 1.9 when first and second derivatives are computed. BAE decreases the LCPO average absolute atomic error from about 2.3 Ų to about 1.7 Ų (average for larger compounds). BAE was introduced into the MacroModel molecular modeling package and tests show that it increases the efficiency of first- and second-derivative energy minimizations and molecular dynamics simulations without adversely affecting the stability or accuracy of the calculations. BAE parameters were developed for the most important atom types in biopolymers, based on a parameterization set of 18 compounds of different size (33–4346 atoms) and class (organics, proteins, DNA, and various complexes), consisting of a total of 23,186 atoms. ©1999 John Wiley & Sons, Inc. J Comput Chem 20, 586–596, 1999     

Flamelet-Modeling of Inverse Rich Diffusion Flames

An experimental and numerical investigation of a confined laminar inverse diffusion flame (IDF) with pure oxygen as oxidizer and carbon dioxide diluted methane as fuel with a global stoichiometry of partial oxidation processes (equivalence ratio of 2.5) is presented. The present burner setup allows studying both the flame and the post-flame zone in a simplified geometry considering typical operating conditions as found in large-scale gasifiers. This partial oxidation flame setup is characterized by very high temperatures close to the stoichiometric oxidation zone due to oxy-fuel combustion, whereas lower temperatures and slow endothermic post-flame conversion reactions with long residence times are found in the fuel rich post-flame region. The scope of this paper is to investigate different modeling approaches suitable for both regimes by comparing the simulation results to detailed experimental data. Planar OH laser-induced fluorescence (OH-LIF) was performed for measuring the hydroxyl radical in the reaction zone and the results are compared to CFD calculations. Based on this comparison, the necessary level of detail of diffusion flux modeling, which includes Soret and Dufour effects, is analyzed and established. Finally, steady and unsteady non-premixed flamelet approaches based on a single mixture fraction are used in order to study their applicability for both the oxidation and post-flame zone. Significantly different time scales are obtained using different flamelet paths. Their influence on the results is investigated in the steady flamelet and the Lagrangian flamelet approach.     

Visualization of Protein‐Specific Glycosylation inside Living Cells

Protein glycosylation is a ubiquitous post‐translational modification that is involved in the regulation of many aspects of protein function. In order to uncover the biological roles of this modification, imaging the glycosylation state of specific proteins within living cells would be of fundamental importance. To date, however, this has not been achieved. Herein, we demonstrate protein‐specific detection of the glycosylation of the intracellular proteins OGT, Foxo1, p53, and Akt1 in living cells. Our generally applicable approach relies on Diels–Alder chemistry to fluorescently label intracellular carbohydrates through metabolic engineering. The target proteins are tagged with enhanced green fluorescent protein (EGFP). Förster resonance energy transfer (FRET) between the EGFP and the glycan‐anchored fluorophore is detected with high contrast even in presence of a large excess of acceptor fluorophores by fluorescence lifetime imaging microscopy (FLIM).